A polymer electrolyte fuel cell (or proton exchange membrane fuel cell) has advantages such as a long life, a high output, less degradation due to activation/suspension, low operating-temperature, and easy activation/suspension.
For those advantages, the fuel cell is expected to be used in a wide range of applications including a power source for a portable device such as a digital camera, a digital camcorder, a mobile phone, or a notebook personal computer, a power source for an electric automobile, and a distributed power source for business or household use.
Of those applications, the portable device (for example, digital camera) having the polymer electrolyte fuel cell mounted thereon is advantageous in that the it can be used for a longer period of time than that of a portable device having a lithium ion battery, which is mainstream now, mounted thereon.
Recently, since the power consumption of the portable device increases steadily, and therefore since there is a fear that use of only the lithium ion battery may lead to power shortage, there is a need for rapid development of a fuel cell with a high efficiency. For this reason, in the technical field of the fuel cell, various improvements are being made in a membrane electrode assembly, a cell structure, power generation conditions, and the like.
Meanwhile, in a power generation system having the polymer electrolyte fuel cell mounted thereon, there are cases where lowering of output may be caused by repeated activation and suspension according to the conditions.
In particular, at the time of suspension of the power generation system, when a power generation reaction inducing gas such as a fuel gas or oxidizer gas remains in the fuel cell, there is a fear of causing degradation such as lowering of output voltage and corrosion of constituting components.
For that reason, hitherto, as disclosed in Japanese Patent Application Laid-Open No. H02-33866, there has been proposed an operation suspension method for a power generation system in which an inert gas is used to purge a power generation reaction inducing gas in a phosphoric acid fuel cell system. In the operation suspension method, in order to prevent the degradation of the fuel cell during suspension thereof, a fuel remaining in a fuel path is purged by nitrogen, while an oxidizer electrode and a fuel electrode are short-circuited, whereby oxygen dissolved in phosphoric acid impregnated in an electrolyte layer is removed.
At that time, in order to bring the oxidizer electrode and the fuel electrode into a short-circuited state and then into open-circuit state, a timer is utilized.
Further, Japanese Patent Application Laid-Open No. H10-144334 discloses that at the time of suspension of the operation of a fuel cell, an inert gas (nitrogen gas) is supplied to each of an anode and a cathode to remove hydrogen and oxygen remaining in the cell, while a dummy resistor is connected between the electrodes to cause short circuiting.
On the other hand, in order to attain space saving and size reduction of a power generation system, adoption of an operation suspension method using no inert gas is advantageous. Therefore, in Japanese Patent Application Laid-Open No. 2003-115305, it is proposed that an external resistor which allows a feeble current to flow is connected to each pair of separators of a fuel cell.
Japanese Patent Application Laid-Open No. 2003-115305 discloses that an external resistor is provided with a switch, and when the operation of the fuel cell is suspended, the switch of the external resistor is closed, and a feeble electric current is allowed to flow between each pair of separators of a fuel cell, thereby consuming a residual gas.
In addition, the fuel cell has an extremely small output at low temperatures, so that a desired output cannot be obtained. Therefore, it is required to warm up the fuel cell.
Japanese Patent Application Laid-Open No. 2003-115305 discloses that after the suspension of the operation of the fuel cell utilizing the above-mentioned method, by keeping the switch in a closed state until subsequent activation, warm up for the subsequent activation can be performed. Further, Japanese Patent Application Laid-Open No. 2003-109636 discloses a structure in which a short-circuiting circuit for short-circuiting electrodes of a fuel cell unit is provided, and by causing short circuiting of the short-circuiting circuit, warm up of the fuel cell is performed. At that time, when the temperature becomes less than a predetermined temperature, the short-circuiting circuit is closed, and when the temperature becomes not less than the predetermined temperature, the short-circuiting circuit is opened.
However, there are the following problems with the conventional technique of preventing the fuel cell from being degraded during suspension of the operation thereof, and of performing warm up at the time of activation at low temperatures.
In the fuel cell to be mounted on a portable device, the space for housing the fuel cell is limited, so that it is required to adopt a control unit which is as compact as possible.
In contrast, in the fuel cell systems as disclosed in Japanese Patent Application Laid-Open Nos. H02-033866 and H10-144334, in which the fuel remaining in the fuel path is purged by use of nitrogen, a purge gas supply unit is additionally required.
Consequently, with the above-mentioned conventional technique, there arises a disadvantage in reducing the size of the fuel cell system.
Further, the technique of performing warm up at the time of activation of the fuel cell at low temperatures disclosed in Japanese Patent Application Laid-Open No. 2003-109636 above has a problem of being influenced by the temperature of use environment or the like.
That is, in this technique, for opening/closing the short-circuiting circuit operated at the time of warm up, a variable resistive element whose resistance value reversibly changes based on the element temperature is used. Therefore, the technique has a problem that the operation conditions depend on the temperature during use.